1. Field of the Invention
The invention relates generally to well logging of Earth boreholes and, more particularly, to a method and apparatus for determining the properties of subsurface rock such as fluid-pressure diffusivity of porous media.
2. Background Art
U.S. Pat. No. 3,599,085 describes a method in which a sonic source is lowered down a borehole and used to emit low frequency sound waves. Electrokinetic effects in the surrounding fluid-bearing rock cause an oscillating electric field in this and is measured at least two locations close to the source by contact pad touching the borehole wall. The ratio of the measured potentials being related to the electrokinetic skin depth to provide an indication of permeability of the formation.
U.S. Pat. No. 4,427,944 describes a method which injects fluid at high pressure of alternating polarity to the formation and measurement of the generated transient streaming potentials in the time domain to estimate the characteristic response time which is inversely proportional to the formation permeability.
U.S. Pat. No. 2,814,017 describes a method where by measuring of the difference in phase between periodic pressure waves passed through a formation and potentials generated by the oscillatory motion of the formation caused by these pressure waves and, conversely, application of a periodically varying electric current to the formation fluid to generate periodic pressure waves in the formation by electro-osmosis. Measurements of the phase shift in the frequency domain between the generating and generated quantities is said to be a measure of permeability of formation.
U.S. Pat. No. 5,417,104 describes a method whereby sound waves of fixed frequency are emitted from a downhole source and the resulting electrokinetic potentials measured. An electrical source of fixed frequency is then used to produce electro-osmotic signals and the acoustic response measured. Using both responses together, the permeability is then deduced, provided the electrical conductivity of the rock is also separately measured.
U.S. Pat. No. 4,904,942 describes several arrangements for recording electrokinetic signals from subsurface rocks mainly with the electrodes measuring the signals at or close to the earth's surface but including use of acoustic source mounted on a downhole tool. There is no indication of permeability being deduced. A further related (inverse) method is described in U.S. Pat. No. 5,877,995, which contains several arrangements for setting out electrical sources and acoustic receivers (geophones) in order to measure electro-acoustic signals induced in subsurface rocks.
U.S. Pat. No. 6,225,806 describes an apparatus for enhancing the seismo-electric measurements where a seismic source with two frequencies radiates radially a seismic signal within the borehole and the electric signals are recorded by a pair of electrodes above and below the seismic source. This type of measurement is said to determine the permeability of the formation. These methods are unable to measure continuously the permeability of porous rock directly with accuracy from a downhole tool. Streaming potential measurements cannot cross easily the mudcake pressure barrier and cannot be used as a continuous logging tool as it needs to impose a big differential pressure and requires time for the pressure front to diffuse into the formation.
Electro-osmotic measurements are closely related to permeability see (U.S. Pat. No. 5,417,104); however, measuring the induced pressure field through the mudcake with a pressure gauge is not feasible because the mudcake is a pressure barrier.